1. Field of the Invention
The present invention relates to an evaporative emission control system which prevents emission of fuel vapor in the liquid fuel tank (hereinafter, "liquid fuel" will be referred to as "fuel") to atmosphere. Specifically, the present invention relates to an evaporative emission control system which is capable of detecting failure occurring in the system.
2. Description of the Related Art
An evaporative emission control system which prevents evaporative emission from internal combustion engines is commonly used in automobile engines. Usually, an evaporative emission control system includes a canister containing an adsorbent such as active carbon which adsorbs fuel vapor in a fuel tank of the engine. In such a system, a flow of purge air through the canister is established when the engine is operated at predetermined operating conditions in order to prevent the adsorbent from being saturated with adsorbed fuel vapor. The purge air supplied to the canister causes the adsorbent to release adsorbed fuel vapor, and the purge air after passing through the canister (which contains fuel vapor, and called "purge gas" hereinafter) is supplied to an intake air passage of the engine to burn fuel vapor contained in the purge gas in the combustion chambers of the engine.
If failure occurs within such an evaporative emission control system, fuel vapor in the fuel tank is not supplied to the engine and the fuel vapor is discharged to atmosphere, thus air pollution occurs. For example, when leakage occurs from the canister or fuel tank, or connecting piping between the canister and the fuel tank, or between the canister and the intake air passage of the engine, the fuel vapor is discharged from the leaked portions to atmosphere. Further, even if the leakage occurs in such portions, the driver of the automobile does not notice that the failure has occurred, and may continue the operation of the automobile. Therefore, various failure detecting devices are used to announce the failure in the evaporative emission control system.
An example of such a failure detecting device is disclosed in Japanese Unexamined Patent Publication No. 6-108930. The device in the '930 publication comprises an internal pressure control valve which is disposed in a fuel vapor passage connecting the canister and the fuel tank for controlling the flow rate of the fuel vapor in the fuel vapor passage, and a pressure detecting device which is capable of separately detecting the pressures in the fuel vapor passage at a portion upstream (i.e., the fuel tank side) of the internal pressure control valve and at a portion downstream (i.e., the canister side) of the same. The failure detecting device in the '930 publication determines whether failure occurs in the fuel tank side of the system or in the canister side of the system separately based on the pressures upstream and downstream the internal pressure control valve, detected by the pressure detecting device.
In the '930 publication, the pressure detecting device consists of a single pressure sensor which is, via a three-way switching valve, connected to the portions of the fuel vapor passage upstream and downstream of the internal pressure control valve. Therefore, the pressure sensor can be selectively connected to the upstream portion and the downstream portion of the internal pressure control valve. This enables the device in the '930 publication to detect failure of the system in the upstream side and the downstream side of the internal pressure control valve using a single pressure sensor.
However, a problem arises when the pressures of the fuel vapor passage upstream and downstream of the internal pressure control valve are used separately for detecting failure in the canister and the fuel tank. When detecting a failure of the evaporative emission control system based on the pressure detecting device, the detected pressure value itself cannot be used for failure detection. That is, a smoothed pressure value, which is obtained by smoothing fluctuations of the pressure value detected by the pressure detecting device, must be used for failure detection to eliminate the possibility of error in the detection. During the operation of the engine, the pressure values of the canister and the fuel tank which are detected by the pressure sensor fluctuate, largely due to the vibration of the engine or the movement of fuel in the fuel tank. Further, when failure of the evaporative emission control system is detected based on the pressure in the canister and the fuel tank, the threshold pressures used for the failure detection are relatively small. Therefore, if the raw pressure values (pressure values detected by the pressure sensor) are used for detecting failure in the system, error may occur due to fluctuation of the raw pressure values (i.e., noise in the detected pressure values). To prevent this from occurring, the failure detection of the evaporative emission control system is usually performed based on the smoothed pressure values which are obtained by smoothing fluctuations of the pressure values detected by the pressure sensor.
However, when smoothing fluctuations of the raw pressure values detected by the pressure sensor, further errors may occur if the raw pressure values of the canister and the fuel tank have their fluctuations smoothed to the same degree. The fluctuations of the pressures in the canister and the fuel tank are quite different in their period of cycle and amplitude. Therefore, if both the pressure values of the canister and the fuel tank are smoothed to the same degree, there is a possibility that the pressure change which must be detected for detecting a failure is smoothed, or that fluctuations of the pressures which must be ignored in the failure detection are detected and used for the failure detection.
Therefore, when performing the failure detection of the evaporative emission control system based on the pressures in the canister and the fuel tank, it is necessary to smooth the fluctuations of the detected pressure values of the canister and the fuel tank to an extent in accordance with the respective characteristics of the fluctuations of the pressures in the canister and the fuel tank.
However, in the '930 publication, no consideration is given to this problem. Further, since a single pressure sensor is used for detecting the pressures in both the canister and the fuel tank in the '930 publication, it is naturally considered that the pressure values detected in the canister and the fuel tank both have their fluctuations smoothed to the same degree. Therefore, the '930 publication does not disclose the solution to the above problem.